[feature] Position camera in from of plane (not using the stack world bbox)

First of all, thank you for releasing such an impressive viewer!

I need to fit the camera to the containing element, but I can't find a way to do it.
Take for example lesson #3 of your tutorial: how can I set the size of the camera such that it occupies the maximum available space (keeping the proportions, of course...). I played around with zoom but I can't find a way to determine the zoom factor based on container size.

Start with lesson #3 of the tutorial and trace how the camera, zoom, and containing element are configured. Determine how camera sizing can use the container while preserving proportions and position it in front of the plane rather than using the stack world bounding box; done means the viewer fills the available container space correctly.

Written by the indexing model from the issue text.